1. Field of the Invention
This invention relates to a process for the preparation in hexane or related saturated aliphatic hydrocarbon of a polybutadiene having at least 70% cis-1,4 and less than 10% 1,2 microstructure. More specifically, the process involves the polymerization of 1,3-butadiene in the presence of a novel catalyst composition.
2. Related Prior Art
Halasa U.S. Pat. No. 3,993,856 describes the polymerization of butadiene to high cis-1,4 polymers using a catalyst composition comprising (a) a cobalt component, (b) an RAlX.sub.2 reducing component wherein R is H or hydrocarbyl, and X is halogen, and (c) CS.sub.2. In column 2, lines 3-13, reference is made to the fact that aromatic solvents, such as benzene and toluene, are preferred and that while aliphatic solvents, such as hexane, etc., may be used, the cis content of the product is decreased by their use. Moreover, lines 14-24 point out that even with aromatic solvents, the cis-1,4 content drops off when the polymerization temperature is above 40.degree. F.
Because of its toxicity the CS.sub.2 modifier is difficult and dangerous to use and requires expensive precautions to avoid even minimal amounts escaping into the atmosphere. Moreover, benzene and toluene are also toxic and it would be preferable to use the less toxic hexane as a solvent both for its decreased toxicity and because of the lower expense involved in its recovery and reuse.
U.S. Pat. No. 3,203,945 shows in its Example III (Cols. 8 and 9) that when heptane is substituted for toluene as the solvent in similar polymerizations using dipyridinocobaltous chloride and ethyl aluminum dichloride, the conversion is 10% less and the molecular weight of the product is much lower than when toluene is used.
U.S. Pat. No. 3,135,725 in a general description of solvents makes reference to hexane and heptane but uses only benzene and xylene in its examples and thereby implies that they are preferred probably for the reasons shown in U.S. Pat. No. 3,203,945.
U.S. Pat. No. 3,066,127 in describing a similar process, shows a preference for aromatic solvents and only refers to aliphatic hydrocarbons as being suitable when used in combination with aromatic or cycloaliphatic hydrocarbons.
It is desirable therefore that a polymerization system suitable for use in hexane or related saturated aliphatic hydrocarbon should be developed for the production of satisfactory high cis-1,4 polybutadiene in high yields to avoid the toxicity, dangers and expense involved in the use of benzene, toluene, CS.sub.2, etc.